Electrical-field-flow-fractionation (E-FFF) and dielectrophoretic-field-flow-fractionation (DEP-FFF) are known in the art. For example, U.S. Pat. No. 5,240,618 discloses an electrical field-flow-fractionation method, and U.S. Pat. Nos. 5,888,370, 5,993,630 and 5,993,632 disclose methods and apparatuses for fractionation using conventional and generalized dielectrophoresis and field flow fractionation. In E-FFF (Caldwell and Gao, 1993), electrophoretic forces are used to balance sedimentation forces (for large particle application, particle size being several micron or larger) and/or diffusion forces (for small particle application) and to control particle equilibrium positions (or equilibrium distribution profile) in a fluid flow velocity profile. Particles of different charges or sizes or densities exhibit different equilibrium positions (or different distribution profiles), and are caused to move through the chamber at different velocities, and can thus be separated into different fractionations. In DEP-FFF (Huang et al, 1997, Markx et al, 1997, Wang et al, 1998), DEP force components in the vertical direction are used to balance sedimentation forces and control particle equilibrium positions in a fluid flow profile. Particles of different dielectric properties are positioned at different heights in the flow profile and are thereby transported at different velocities. A particle mixture introduced into an E-FFF or DEP-FFF chamber can be fractionated into sub-populations according to the time they exit the chamber. E-FFF separation has been demonstrated on colloidal adsorption complexes. DEP-FFF separation has been demonstrated on synthetic polystyrene beads and biological cells.
However, the currently available apparatuses and methods used in field-flow-fractionation suffer from the following limitations. For E-FFF, electrode polarization presents a significant problem since majority of the applied voltage is dropped across the electrode/medium interface. Furthermore, electrical charge may be used as a separation or fractionation parameter for only certain cases. Similarly, for DEP-FFF, dielectric properties may be used as separation bases for only certain problems. Positive DEP force has not been exploited for particle DEP-FFF separation. The separation efficiency using current field-flow-fractionation methods is still not satisfactory for many application problems. Thus, there is a need to further improve field-flow-fractionation methods so that the methods have improved applicability and separation efficiency to many separation problems. The present invention addresses these and other related needs in the art.